I. Technical Field
The present invention relates to a method and apparatus for making uncured thermoset pellets and the pellets made thereby. More particularly, the invention relates to microsphere-filled thermoset resin pellets which are extruded in paste form, gelled and pelletized according to a unique method with a specialized production apparatus.
II. Background Art
Plastic materials are currently used for filling and reinforcing structural members. Expanded polyurethane foam is known to be used for filling structural members to improve sound dampening, thermal insulating and crush strength qualities of the structures. Plastic fillings are used in boats to fill flotation cavities and in vehicles to act as sound baffles and reinforcements for hollow structural members.
The most common type of plastic used in such applications is expanded polyurethane foam. In structural reinforcement applications, expanded polyurethane foam lacks compressive and tensile strength and has extremely low heat resistance.
In recent years, specialized plastic reinforcements have been developed wherein macropheres are formed of glass microspheres which are combined with a phenolic binder. The macrospheres are then coated with a phenolic resin which increases the strength and shell thickness of the macrospheres but also adds weight to the final product. After coating with a phenolic resin, the macrospheres are coated with a B-staged phenolic or epoxy which permits the macrospheres to be bound together to form a structural reinforcement. Examples of two types of such macrospheres are two materials manufactured by 3M Company and identified by the following trade designations: M27X for uncoated macrospheres and M35EX for phenolic-coated macrospheres. The above macrospheres are known to be used as structural reinforcements for vehicles.
Another approach to improving the performance of plastic fillers to function as reinforcements is to provide styrofoam beads which are coated with an epoxy. One such product is sold by W. D. Grace Company under the tradename Ecosphere. The styrofoam bead has an epoxy coating which is in the form of a cured shell. The styrofoam bead with cured shell may be coated with an adhesive and used as a constituent element for structural fillers. However, the coated styrofoam beads are expensive and have only slighter greater compressive strength than polyurethane fillers and have only limited heat resistance due to the fact that the styrofoam substrate may begin to melt at temperatures as low as 210.degree. F.
In terms of processing techniques, it is known to extrude thermoset materials by first B-staging the thermoset materials by heating them prior to extrusion. The B-staged thermoset materials emulate thermoplastic materials and are extrudable to a limited extent. However, the high viscosity of B-staged thermoset materials prevents incorporation of a high percentage of microsphere fillers since the heat and friction developed during the mechanical mixing of the B-staged resin causes the microspheres to be crushed and would limit the weight savings sought to be realized by the incorporation of lightweight microsphere fillers.
Prior art plastic reinforcements fail to provide a lightweight yet strong reinforcement which is thermally stable and competitive in cost to other types of structural reinforcements. These and other problems and disadvantages are overcome by the present invention as summarized below.